Rev. Notes > AS > Physical quantities and units Previous Topic Physical quantities and units Next Topic Topics
 Physics Areas of Physics Interdisciplanary areas of physics Quantities in Physics Base Units Convention for indicating units Derived Units S.I. Units (International System of Units) Supplementary units Homogeneity of equations Significant figures Rules of Significant figures Further discussion on Significant figures Dimensions Scientific notation The Avogadro's Constant Scalars and Vectors Addition and subtraction of co-planar vectors Rectangular components of a vector

## Quantities in Physics

The foundation of physics rests upon physical quantities in terms of which the laws of physics are expressed. Therefore, these quantities have to be measured accurately. Among these are mass, length, time velocity, force, density, temperature, electric current, and numerous others.
Physical quantities are often divided into two categories: base quantities and derived quantities.

Derived quantities are those whose definitions are based on other physical quantities. Velocity, acceleration and force etc. are usually viewed as derived quantities.

Base quantities are not defined in terms of other physical quantities. The base quantities are the minimum number of those physical quantities in terms of which other physical quantities can be defined. Typical examples of base quantities are length, mass and time.
The measurement of a base quantity involves two steps: first, the choice of a standard, and second, the establishment of a procedure for comparing the quantity to be measured with he standard so that a number and a unit are determined as the measure of that quantity.

An ideal standard has two principal characteristics: it is accessible and it is invariable. These two requirements are often incompatible and a compromise has to be made between them.

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